1. Field of the Invention
This invention relates to a process for treating waste water containing organic and/or inorganic oxidizable substances by wet oxidation with a use of a catalyst performed in the presence of an oxygen-containing gas, and more particularly pertains to a process for treating waste water efficiently for a long period in a stable manner by using a solid catalyst containing activated carbon and by regulating the oxygen concentration in an exhaust gas.
This invention further relates to a process for suppressing deterioration of the catalytic activity of the solid catalyst at the time of temperature rising when starting up a operation of the wet oxidation and/or at the time of temperature lowering when suspending the operation by protecting the catalyst according to needs, or to a process for efficiently recovering the degraded catalytic activity of the solid catalyst containing activated carbon according to needs.
2. Description of the Related Art
Heretofore, there have been known, for example, biological treatment and wet oxidation treatment, as means for purifying waste water containing organic or inorganic oxidizable substances. Biological treatment has a disadvantage in consuming a long time to decompose the oxidizable substances in the waste water. Further, this treatment is limited to treat waste water of a low concentration. In the case where the waste water is of high concentration, it is required for diluting the waste water to a proper concentration. The method demands a large space for the installation of facilities for diluting waste water. Further, microorganisms that are used in the biological treatment are susceptible to change of the environment such as temperature. These factors make it difficult to stably operate the waste water treatment for a long time.
Wet oxidation is a process for treating waste water in the presence of oxygen at a high temperature under high pressure to oxidize and/or decompose oxidizable substances in the waste water. As an example of this process, there has been proposed a wet oxidation using a solid catalyst (herein after may be referred to as xe2x80x9ccatalytic wet oxidationxe2x80x9d) as a means for speeding up the reaction rate and loosening the requirements for reaction (reaction condition). In the catalytic wet oxidation, a catalyst using an oxide, and a catalyst using combination of such an oxide and a precious metal element are employed.
In the aforementioned wet oxidation process, it has been required to treat waste water at a temperature of 170xc2x0 C. or more in order to oxidize and/or decompose various oxidizable substances in the waste water. It is often the case that a pressure as large as 1 MPa (Gauge) or more is required. For example, Japanese Unexamined Patent Publication No. 11-347574 proposes a technique in which a catalyst comprising platinum supported on titania is used and acetic acid is subjected to wet oxidation at 170xc2x0 C. This technique still requires a treating condition of setting the temperature at a relatively high degree. Therefore, there has been a demand for developing a technique of treating waste water at a low temperature and low pressure and with a high treating performance.
In view of the above problems residing in the prior art, the inventors of this invention has been developing a new catalyst and researching a new waste water treating process. As a result of trials and errors, the inventors found that a solid catalyst containing activated carbon exhibits significantly high catalytic activity to organic and/or inorganic oxidizable substances under a temperature lower than 170xc2x0 C. and a low pressure. Such a low temperature and low pressure condition contributes to loosening the requirements for reaction.
In the case where a solid type catalyst containing activated carbon is used, there has to be considered various problems as follows. Activated carbon is liable to be brought into combustion under the conventional wet oxidation. Therefore, it has been impossible to utilize activated carbon as a catalytic component for wet oxidation. Specifically, in the case where a catalyst containing activated carbon is employed, it is often the case that the catalyst does not have heat resistance of resisting a high temperature such as 170xc2x0 C. or more. Even if the catalyst exhibits a high catalytic activity at an initial stage of reaction, the catalytic activity deteriorates rapidly within 100 hours or less. Therefore, utilization of the catalyst containing activated carbon has been practically impossible.
On the other hand, in the case where there is not provided sufficient measures for protecting the catalyst during its use even at a low temperature such as 170xc2x0 C. or less, the activated carbon itself is subjected to combustion due to existence of oxygen containing gas. Consequently, the catalytic activity also deteriorates within a short period such as within one hundred to several hundreds hours. Thus, utilization of the activated-carbon-containing catalyst was impossible in the conventional wet oxidation for treating waste water.
Japanese Unexamined Patent Publication No. 11-179378 discloses a technique of oxidizing an oxygen-containing organic compound having solely one carbon atom per molecule at a temperature of 100xc2x0 C. or lower with use of a catalyst in which a precious metal is supported on activated carbon. This technique is not applicable to treating an organic compound having two or more carbon atoms per molecule or inorganic compound. Also, this publication does not fully consider durability (heat resistance) of the catalyst in which a precious metal is supported on activated carbon.
The above problems such as a possibility of combustion of activated carbon itself and deterioration of catalytic activity were also observed when treating waste water containing organic and/or inorganic oxidizable substances at the time of temperature rising by starting up the operation of the wet oxidation apparatus and/or at the time of temperature lowering by suspending the operation if the treatment is performed under the same oxidization atmosphere as in the conventional system.
It is often the case that raising the pressure in the apparatus is required so that the waste water retains its liquid phase while raising the temperature of the waste water in the wet oxidation. Therefore, it is a general practice to supply oxygen-containing gas even in the absence of oxidizable substances in order to maintain the pressure in the apparatus to a certain level at the time of starting-up the apparatus. As a result, it is likely that the catalytic activity of the catalyst containing activated carbon is deteriorated at the time of starting up the operation of the apparatus prior to actual treatment of waste water as well as during suspension of the operation of the apparatus, namely, suspension of supply of the waste water. In order to avoid such a problem, there has been proposed a technique of supplying gas which does not contain oxygen. e.g., nitrogen gas, into the apparatus during starting-up operation of the apparatus or suspension of the operation of the apparatus. This technique, however, is not desirable in the aspect of cost performance and necessity of cumbersome operation. There has also been a problem that the catalytic activity is lowered due to existence of oxygen that has remained in the apparatus or adsorbed to the catalyst even during non-supply period of oxygen-containing gas.
Japanese Unexamined Patent Publication No. 4-300696 discloses a technique of initiating an operation of the apparatus for catalytic wet oxidation. In this publication, disclosed is a technique of omitting or simplifying a device that is required for pre-heating of the apparatus during start-up operation, rapidly initiating oxidation, and treating waste water with high performance. This publication, however, does not propose a technique of suppressing deterioration of catalytic activity.
Maintenance of high catalytic activity for a longer period and carrying out the waste water treatment with high performance have been strongly demanded recently. For instance, in the case where the waste water contains oxidizable substances which are hard to decompose, it is required to set the treating temperature at a relatively high level in order to accomplish waste water treatment with high performance. In such a case, durability of the activated-carbon-containing catalyst is liable to deteriorate.
Activated carbon has a property of absorbing oxidizable substances in the waste water. In the case where the decomposing rate of the oxidizable substances that have been adsorbed to the activated carbon is extremely slow, oxidation and decomposing ability of the catalyst gradually decrease during waste water treatment. Further, in the case where waste water treatment is performed for a long period, a problem involved in waste water treatment such as disorder of the apparatus and erroneous operation of the apparatus cannot be avoided, which may deteriorate the catalytic activity of the activated-carbon-containing catalyst.
In view of the aforementioned various problems, there has been a demand for a technology of recovering the catalytic activity of the activated-carbon-containing catalyst that has been once deteriorated.
Several catalyst recovering techniques have been proposed heretofore. For example, Japanese Examined Patent Publication No. 3-66018 proposes a technique of combining (a) acid washing process and (b) liquid phase reduction process or combining (a) acid washing process and (c) gaseous phase reduction process. The acid washing is such that the catalyst is washed in an acidic aqueous solution containing at least one component selected from the group consisting of hydrochloric acid, nitric acid, phosphoric acid, acetic acid, and propionic acid. The liquid phase reduction is such that the catalyst is reduced with use of an aqueous solution containing at least one component selected from the group consisting of hydrazine hydrate, formaldehyde, sodium borohydride, lithium aluminohydride, sodium tartrate, glucose, potassium formate, and sodium formate. The gaseous phase reduction is such that the catalyst is reduced with use of a gaseous reducing agent containing hydrogen and/or carbon monoxide.
The above mentioned method, however, is not sufficient in recovering the catalytic activity of the activated-carbon-containing catalyst. Conversely, this process may likely to cause deterioration of catalytic activity. Further more the gaseous phase reduction employing hydrogen or carbon monoxide also has a problem because it is difficult to implement the reduction in a state that the catalyst is filled in a waste water treating apparatus as itself. To carry out the water treating process while employing the gaseous phase reduction, it is necessary to take out the catalyst from a reactor and install a furnace exclusively used for reduction and calcination. Such an arrangement, however, is practically unexecutable.
Japanese Examined Patent Publication No. 4-45214 proposes a technique of rendering the catalyst into contact with an aqueous solution containing formic acid and/or oxalic acid at a temperatures ranging from 40 to 85xc2x0 C. and heating the catalyst to decompose the formic acid and/or oxalic acid, thereby reducing the catalyst. This recovering process, however, does not completely recover the catalytic activity of the activated-carbon-containing catalyst. Conversely, this recovering process may cause deterioration of the catalytic activity and corrosion of the parts constituting the apparatus.
Japanese Unexamined Patent Publication No. 9-10602 proposes a technique of contacting a solid catalyst whose catalytic activity has been deteriorated due to oxidation resulting from oversupply of oxygen, substantially without supply of oxygen, with a regenerated solution which contains at least one ammonium salt selected from the group consisting of ammonium sulfate, ammonium chloride, and ammonium carbonate, or which contains ammonia and at least one ammonium salt selected from the group consisting of ammonium sulfate, ammonium chloride, and ammonium carbonate and has pH from 3 to 10. This technique is effective to some extent in recovering the catalytic activity of the catalyst containing activated carbon. However, there is room for further developing the technique to achieve more improved recovering performance.
The present invention has been accomplished to solve these problems. Accordingly, an object of the present invention is to provide a catalyst for oxidizing and/or decomposing organic and/or inorganic oxidizable substances in the waste water by catalytic wet oxidation efficiently in a stable manner and the preparation method thereof.
Another object of the present invention is to provide a method for treating waste water efficiently for a long period in a stable manner by wet oxidation using a catalyst containing activated carbon at low temperature and under low pressure.
Further object of the present invention is to provide a method for suppressing deterioration of the catalytic activity of the solid catalyst at the time of temperature rising when starting up the operation of the wet oxidation and/or at the time of temperature lowering when suspending the operation of the wet oxidation.
Still further object of the present invention is to provide a method for efficiently recovering the deteriorated catalytic activity of the catalyst containing activated carbon.
The object mentioned above is accomplished by a catalyst for the treatment of a waste water, which catalyst comprises activated carbon, (a) component (also referred to as xe2x80x9cfirst componentxe2x80x9d) and (b) component (also referred to as xe2x80x9csecond componentxe2x80x9d).
(a) component is at least one selected from the group consisting of Ti, Zr, Hf, Nb, Ta, Fe, Co, Mn, Al, Si, Ga, Ge, Sc, Y, La, Ce, Pr, Mg, Ca, Sr, Ba, In, Sn, Sb and Bi.
(b) component is at least one selected from the group consisting of Pt, Pd, Rh, Ru, Ir and Au.
The present inventive catalyst can be prepared by a method for the production of the catalyst which method comprises the steps of depositing (a) component on the activated carbon and depositing (b) component on the activated carbon.
Another object of the present invention can be accomplished by a method for oxidizing and/or decomposing organic and/or inorganic oxidizable substances in waste water by wet oxidation with a use of a catalyst, wherein the oxidizable substances are oxidized and/or decomposed with an oxygen containing gas in the presence of the catalyst under pressure such that said waste water retains the liquid phase thereof at temperature of 50 to less than 170xc2x0 C., the catalyst contains activated carbon and controlling an oxygen concentration in an exhaust gas in the range from 0 to 5 vol %.
Further object of the present invention can be accomplished by the method which is characterized in that a catalyst protection liquid which contains easily decomposable substances is supplied at the time of temperature rising when starting up a operation of the wet oxidation and/or at the time of temperature lowering when suspending the operation.
Still further object of the present invention is accomplished by the method which is characterized in that a catalyst recovering liquid which contains easily decomposable substances is supplied to the catalyst under temperatures in the range from 55xc2x0 C. to less than 200xc2x0 C.